Plug Color Vs. AFR

[Guest]

I've got a wideband O2 sensor on my bike and thought this would be some fun information to share. I cleaned some plugs, then set out on a 30 mile ride or so. Mostly cruising between 50-75 with 1 or 2 full throttle runs through 2nd and 3rd gear. My O2 was reading around 13.5:1 in cruise and about 12.0:1 at full throttle. After a lot of playing around I think these are pretty close to ideal numbers given my setup and riding conditions. I'll attach a picture so you can see what these ratios correspond to in plug color. I only happened to snap a pic of cylinders 2-3, but the others look pretty much the same.

Just for some more background info this is on a 78 GS750 with a 4 into 1 and stock airbox. Also worth mentioning the engine is tired and has slightly low compression with the crankcase ventilation still hooked up to the airbox.
Currently running a 102.5 main, 17.5 pilot (not really needed my fuel screws are just barely off the seat), stock needle position and the side mixture screws 1.25 ish turns out

 

[Grimly]

That doesn't surprise me these days. It's hard to get the classic biscuit-brown colour any more with the witch's brew of modern fuel. All there seems to be is shades of grey.

 

[aclaytonb]

I couldn't get any browning on my GS550 plugs until the mains hit 95 and the pilot hit 17.5 (airbox lid removed, recommended 4-1 settings from VH).

Everything with stock settings look like yours or whiter.

 

[KEITH KRAUSE]

Just a note about your jetting. The 17.5 pilot jets should be replaced with the stock 15. The 4-1 exhaust won't have a significant effect on the pilot circuit jetting when you still run the stock intake. The 17.5 jets will make it run richer and build up carbon, especially with low compression. The side air screws should always be adjusted using the highest rpm method, beginning with a base idle of 1,000 rpm. The 4-1 only mod will only require a half size larger main or a full size at the most. If your mains were 100 stock, then 102.5 seems good. The 4-1 will lean out the jet needle circuit but not much. Even a half position higher (using a standard jetting spacer) may be on the rich side. It can be difficult to get the jetting spot on because the stock restrictive intake and a free flowing exhaust are a poor match.

 

[Guest]

Just a note about your jetting. The 17.5 pilot jets should be replaced with the stock 15. The 4-1 exhaust won't have a significant effect on the pilot circuit jetting when you still run the stock intake. The 17.5 jets will make it run richer and build up carbon, especially with low compression. The side air screws should always be adjusted using the highest rpm method, beginning with a base idle of 1,000 rpm. The 4-1 only mod will only require a half size larger main or a full size at the most. If your mains were 100 stock, then 102.5 seems good. The 4-1 will lean out the jet needle circuit but not much. Even a half position higher (using a standard jetting spacer) may be on the rich side. It can be difficult to get the jetting spot on because the stock restrictive intake and a free flowing exhaust are a poor match.

I would agree with you on the 15 pilot jet. I swapped to a 17.5 trying to get rid of a lean spot right around 1/4 throttle. The larger pilot didn't make much of a difference and as a result I now have the bottom fuel screws barely turned out. Ideally i would go back to a 15 but i really have noticed no problems with the small screw openings. Not necessary, but no harm done. You're also spot on with the main jet. Something in between a 100 and 102.5 would probably be perfect at full throttle I'm a touch on the rich side

In my testing of moving the needle by half steps and full clip positions; I've found no need for even a half step movement of the needle. That just translates to lower fuel economy.

It seems one of the downsides of the VM carburetors is this lean spot right around 1/4 throttle. I have tried absolutly everything to get rid of it and i have come up with nothing. I think this is an inherent problem with the VM carburetors. I have always had this gap between the idle circuit and needle taper that I can only control with the side mixture screws. With my gearing that translates to about 55 mph in 5th and makes it cruise leaner than I would like. This can be tuned out with the side mixture screw, but does leave the rest of the throttle positions a little more rich than what is necessary. My best guess is everyone that tunes their bikes without the O2 sensor ends up with a tune like I just mentioned that is a little on the rich side.

What I've done to work around this is set my mixture screws about an extra 1/2 turn out in the winter where I am less worried about running too hot. Turn them back in in the summer to help cool the engine.

 

[TxGSrider]

On my CV style "slingshot" carbs with AFRs similar to yours' my plugs are a little lighter than yours.

I would think that VM carbs will probably have a slight lean spot under any type of acceleration if the AFR is set correctly for steady RPM's... it the nature of a "non CV" style carburetor without an accelerator pump. You can cover some of that lean area under acceleration with a richer set up, but then your AFR at steady RPMS will be rich. That's probably why the factories went to CV's and the aftermarket went to using accelerator pumps (IE RS and FCR carbs).

 

[KEITH KRAUSE]

I would agree with you on the 15 pilot jet. I swapped to a 17.5 trying to get rid of a lean spot right around 1/4 throttle. The larger pilot didn't make much of a difference and as a result I now have the bottom fuel screws barely turned out. Ideally i would go back to a 15 but i really have noticed no problems with the small screw openings. Not necessary, but no harm done. You're also spot on with the main jet. Something in between a 100 and 102.5 would probably be perfect at full throttle I'm a touch on the rich side

In my testing of moving the needle by half steps and full clip positions; I've found no need for even a half step movement of the needle. That just translates to lower fuel economy.

It seems one of the downsides of the VM carburetors is this lean spot right around 1/4 throttle. I have tried absolutly everything to get rid of it and i have come up with nothing. I think this is an inherent problem with the VM carburetors. I have always had this gap between the idle circuit and needle taper that I can only control with the side mixture screws. With my gearing that translates to about 55 mph in 5th and makes it cruise leaner than I would like. This can be tuned out with the side mixture screw, but does leave the rest of the throttle positions a little more rich than what is necessary. My best guess is everyone that tunes their bikes without the O2 sensor ends up with a tune like I just mentioned that is a little on the rich side.

What I've done to work around this is set my mixture screws about an extra 1/2 turn out in the winter where I am less worried about running too hot. Turn them back in in the summer to help cool the engine.

The reason the larger pilot jets "didn't make much of a difference" to fix your lean spot at 1/4 throttle, is because the pilot jets (pilot circuit) is not designed to effect the carburetion at 1/4 throttle position. That's the job of the jet needle/needle jet circuit. There is some overlap between the pilot and jet needle circuits but increasing the pilot jet size won't fix a 1/4 throttle issue. The cut-away on the throttle slides is the transition point to the jet needle. They DO make slides with different cut-away sizes but they're expensive, hard to find now, but most importantly, rarely needed in NORMAL jetting applications. Because you're trying to make a stock, restrictive intake work with a free flow exhaust, you're having this "rare" issue.
Your side air mixture screws are now rendered somewhat useless because the too large pilot jets are passing too much fuel for the pilot fuel screws to regulate and there is too much fuel being supplied to the entire pilot circuit. You'll never get the jetting right with the 17.5 jets. You're wasting fuel too. Raising the jet needles 1/2 position would be the usual thing to do. Personally, I think 1/2 position is a little rich for your set up but because leaving the jet needle in the stock position is on the lean side, I'd go with the little bit rich rather than lean. A standard jet needle jetting spacer used for 1/2 position changes, is approx' .022" thick. If you can find a thinner spacer around .014", that would probably be perfect. I don't believe raising the jet needles 1/2 position with a standard jetting spacer could result in a rich mixture that's serious enough to cause poor mpg's. Not when that adjustment is made on a jet needle circuit that is presently too lean to begin with. You may find that a fix to your issue isn't going to happen because this may truly be an issue that only different slide cut-aways will fix. Install the stock #15 pilot jets regardless of what else you do.
I would install the #15 pilot jets, raise the jet needles with the thinner spacers I described if you can find/make them, or else the standard spacers. BENCH sync the carbs as best you can. Then adjust your pilot fuel screws and side air screws. I'd start with the pilot fuel screws 1 turn out from LIGHTLY seated. I'd set the side air screws at 1 1/2 turns out. Then warm up the bike to full operating temperature, set the idle at 1,000 rpm's (no higher than 1,100 rpm's) by using the throttle adjusting knob. Adjust each side screw for the highest rpm possible, then return the idle to 1,000 rpm's by using the throttle adjuster knob, and go to the next screw. Once the side air screws are adjusted, you should sync the carbs with a vacuum tool if you have one. Once the vacuum sync is done, fine tune the side air screws for highest rpm again. Now go test and see if the pilot fuel screws need any fine tuning. I doubt they will need much, if any. Even 1/8 turn on the pilot fuel screws effects the jetting.

Another thing you could do to better match the stock intake and the free flow exhaust is to remove the airbox lid. Still not as free flowing as quality pod filters but much less restrictive. No jetting kits are designed to be used with the lid off a stock box. If you do this, you WILL need to raise the jet needles at least 1 full position and install larger main jets. Because the intake and exhaust are then better matched, you'll have a better chance at finding jetting that works better. With no lid and better jetting, you'll make a little more power at higher rpm's.